Electrolytic anode

ABSTRACT

AN IMPROVED ANODE FOR ELECTROLYSIS OF BRINE IS COMPRISED OF A CORROSION RESISTANT VALVE METAL SUBSTRATE AND A THIN ADHERENT EXTERIOR COATING CONSISTING ESSENTIALLY OF RUTHENIUM OXIDE AND A CARBIDE, THE CARBIDE BEING INERT TO THE ELECTROLYSIS ENVIRONMENT. AN ESPECIALLY EFFECTIVE ANODE HAS A COATING OF RUTHENIUM OXIDE AND BORON CARBIDE.

United States Patent US. Cl. 117-230 Claims ABSTRACT OF THE DISCLOSUREAn improved anode for the electrolysis of brine is comprised of acorrosion resistant valve metal substrate and a thin adherent exteriorcoating consisting essentially of ruthenium oxide and a carbide, thecarbide being inert to the electrolysis environment. An especiallyelfective anode has a coating of ruthenium oxide and boron carbide.

- The instant application is a division of application Ser.

No. 878,953 filed Nov. 21, 1969,'which is-in turn a continuation-in-partof application Ser. No. 786,407 filed Dec. 23, 1968 and now Patent No.3,616,329.

This invention relates to novel anodes for cells used for theelectrolysis of brines, and more particularly to improved anodescomprised .ofplatinum group metal coated electrolytic valve metals and amethod for obtaining such anodes.

The anodes of the present invention are particularly useful in.cells..used forithenproductionofchlorine and caustic soda by theelectrolysis of an aqueous solution of sodium chloride. In such cellsgraphite anodes are usually used commercially.'Although' the" graphiteanodes are not entirely satisfactory because their wear rates ar highand impurities such as CO are introduced in the products, nosatisfactory'substitutes have yet been found.

Platinum group metal coated electrolytic valve metals have been proposed.as'substitutes for graphite anodes.

These metallic anodes offer several potential advantages over theconventional graphite anodes, for example, lower overvoltage, lowererosion rates, and higher purity products. Theeconomic advantages gainedfrom such anodes, however, must be sufiiciently high to overcome thehigh cost of these metallic anodes. Anodes proposed theretofore have notsatisfied this condition. Therefore commercialization of the platinumgroup metal anodes has been limited.

One problem is the life of the metallic anodes. A factor whichcontributes to shortening the anode life is the 'socalled undercuttingeffect. For economic reasons the low overvoltage precious metal coatingsare very thin films which are'inherently porous. Although theelectrolytic valve metals are substantially corrosion resistant, thevalve metals are slowly attacked through the pores of these coatingscausing undercutting with subsequent loss of the precious metal film,thereby shortening the life of the anodes. 5

Another problem is the loss of precious metal during operation of thecell. Although the loss is gradual, it is costly because the preciousmetals are expensive and because the erosion of thin coatings shortensthe anode life.

3,687,724 Patented Aug. 29, 1972 The loss of precious metal may be frommechanical wear. At the high current densities desirable in commercialinstallations, the increased flow rate of brine and excessive gassingare conducive to such mechanical Wear. In mercury cells a contributingfactor is amalgamation of the precious metals.

Still another problem is providing such coated anodes by an economicallyfeasible method.

It is the object of this invention to provide, by an economicallyfeasible method, metal electrolytic anodes with improved life and lowermetal losses without sacrificing the low overvoltage characteristics ofthe precious metal coating.

In accordance with the present invention a precious metal anode isprovided which has long life and lower precious metal losses due tomechanical wear and amalgamation. The resistance to amalgamation makesthe anode particularly useful in mercury cells. It was a further ad-'vantage of the anodes of this invention that the electrical propertieswere equal and even superior to conventional anodes using a greaterequivalent weight of platinum group metals.

' The anode of the present invention is comprised of a corrosionresistant metal substrate and a coating consisting essentially ofruthenium oxide and a carbide. Generally any carbide may be used that isinert to the environment of the cell. Preferably the carbide should alsohave relatively good electrical conductivity. By way of exemple, thecarbides that may be used are compounds of the elements selected fromthe group B, Si, Ti, Hf, V, Nb, Ta, Cr, Mo, and W, and combinationsthereof.

The carbides that are particularly useful are those of the so-calledgiant molecule covalent type and the interstitial type. The term giantmolecule covalent carbide refers to carbides characterized by completecovalent bonding, which results in high degrees of hardness andinertness. These carbides have a perceptible electrical conductivity.Boron carbide (B C) and silicon carbide (SiC) belong to this class ofcarbides. Chromium carbide (Cr C is also believed to posses some ofthese characteristics. The interstitial carbides "most useful for theanodes of the present invention are those having relativ'ely large metalatoms (radius equal toabout 1.3 A. or

greater) so that the carbon atoms'in the interstices do not appreciablydistort the metallic lattice. Typical i n.

terstiti'al carbides are TiC, VC, NbC, TaC, MoC, and

WC. All of these carbides, both the covalent and intersti-' ments inelectrolysis cells. Examples of suitable corrosion:

resistant valve metals are Ti, Ta, Nb, Hf, Zr, W, Al, and alloysthereof. It is also well known to have the valve metal as a layer on abase metal'such as copper which is a goodconductor but corrosive to theenvironment, and such modificationsare within the scope of thisinvention.

Anodes of this invention are suitably prepared by depositing a'slurry ofa carbide in the form of a finepowder in a liquid medium containingruthenium on a corrosion resistant substrate and then firing the coatingin an oxidizing atmosphere such as air to drive oil the liquid 7 andform an adherent coherent coating of ruthenium oxide and the carbide.The coating may be deposited using the usual techniques such as bybrushing, spraying or dipping. The coating may also be applied byelectrophoresis. Suitably the carbide is present as a powder having aparticle size of no greater than 250 microns. Preferably at least someof the carbide particles have a diameter no greater than about 10microns. The ruthenium is present as a salt, oxide or the metal per se;it is present dispersed as a fine powder or dissolved in the aqueous ororganic medium. When the slurry contains ruthenium as a salt or metal,the coated substrate is heated at a temperature in the range of about400 to 800 C. to convert such ruthenium to ruthenium oxide and to forman adherent coherent coating. The time required to convert the rutheniummetal or salt to the oxide depends on the temperature used. Typicallythe coated substrates are fired in air at 500 C. for five minutes; butlonger firing times are also used. When the slurry contains ruthenium asruthenium oxide, the coating is heated to higher temperatures, e.g.about 1000" C. and higher, for a period of time necessary to sinter theparticles and form an adherent coherent coating.

Alternatively the ruthenium salt, oxide or metal is applied on thesubstrate followed by an application of the finely divided boron carbideand the coated substrate is fired as indicated above. v

It should be understood that conversion of the ruthenium metal and saltsmay not be complete under the firing conditions given. Normally anequilibrium will be reached under which conversion is to predominantlyruthenium oxide and the balance ruthenium. Such materials are within thecontemplation of this invention. It will be noted that the presence ofthe carbide in the ruthenium salt mixture makes it possible to use theruthenium salt in the formulation in a higher concentration thanpreviously possible and still obtain a coating of principally rutheniumoxide. This is one of the main advantages of using the mixed carbidecoating.

It should be further understood that the precursor composition maycontain an additive for improving the adherence, continuity and abrasionresistance of the coating. The use of such additives is well known intheceramic art for forming thin adherent precious metal coatings, and theyare commonly referred to a fluxes. The choice of ingredients for theflux is dictated in part by the composition of the substrate. It is wellknown, for example, to use salts and resinates of bismuth, chromium,lead, cadmium, tin, copper, boron, antimony, titanium, tantalum,silicon, and uranium. The use of such ingredients is well understood bythose skilled in the art of compounding precious metal decoratingcompositions.

' The concentration of ruthenium oxide in the coating ranges from aboutto 90% by weight and the carbide content ratio ranges from about to 95%by weight. With respect to boron carbide, for example, it has been foundthat higher percentages of boron carbide increase the adherence of thecoating without excessive sacrifice of the electrical characteristics.One preferred embodiment contains about 50% ruthenium oxide and 50%boron carbide.

Several applications of the dispersion may be deposited, preferablyfiring at the indicated temperature is performed after each application.

' The following examples are given by way of illustration and not as alimitation of the invention. It will be appreciated that modificationswithin the scope and spirit of the inventoin will occur to those skilledin the art.

The examples show comparative tests in an electrolytic diaphragm cellusing various anodes.

In each anode the substrate is a sheet of commercially pure titanium /6"x 3" x 0.063". The titanium sheets are prepared for coating by etchingin concentrated hydrochloric acid for 18 hours at a room temperature andcleaning in fluoboric acid.

EXAMPLE I Sample A is prepared as follows:

An aqueous paint composed (by weight) of 3.13% boron carbide (milled toa fine particle size, less than 8.2 microns and 50% less than 5.2microns), 6.87% ruthenium chloride, and 90% water is applied to bothsides of a previously prepared titanium sheet. The coated substrate isfired in air at 500 C. for five minutes. This procedure is repeated anadditional six times to give a coating composed of 52.9% Ru0 and 47.1%boron car. bide (by weight). The total weight gain of the sample is0.0263 gram. This deposit contains an amount of- Ru equivalent in weightto a 17 microinch Ru coating. The deposit showed exceptionally goodadheren'ce and coherence. l

Sample B is prepared as follows: 7

A low overvoltage 70% Pt30% Ir coating having a thickness of 27microinches is appliedto one side of a titanium sheet. The coating isapplied from a paint using a known technique of application and firing.

SampleCis prepared as follows 5 I l A low overvoltage RuO layer having athickness equivalent to 17 microinches of Ru, determinedgravimetrically, is prepared from an alcohol based paint containing RuCllinalool and Z-propanol. The coating is converted to Ru0 by heating inair at 500 C. for 10 minutes.

Samples A, B, and C are used as anodes in a. labora' tory scalediaphragm cell for the electrolysis of 25% NaCl solution. The tests arerun at 35 C. and at a current density of 1000 amperes per square foot(a.s.f.). The chlorine overvoltage is determined with 'a conven' tionalLuggin capillary probe, and the resultspare shown in Table I. I Y

aIss The results in Table I show that anode A, the anode of thisinvention, has excellent overvoltage properties; the performance ofanode A surpassed that of anode C, which has a comparable and equivalentthickness of pre% cious metal but does not contain the carbide, and evensurpassed anode B, which has more than one and one half the thickness ofprecious metal but does not contain the carbide. Y

An additional advantage of anode A is that for an equivalent weight ofprecious metal anode A is less expensive than anode B since Ru is :lessexpensive than the Pt-Ir.

A still further advantage of anode A is that the pres ence of a carbidecompound provides an effectively thicker coating for an equivalentweight of precious metal without the boron carbide and the presence ofthe carbide does not adversely affect the electrical characteristics andin fact shows improvement. In'view of thegreater thicle ness it could beexpected that the coating would have longer life in commercial operationas illustrated in the next example. a I

EXAMPLE u rent density of a.s.f. for 210 hours. shown in Table II.

I TABLE rr' After 210 hours, Sample C would not draw the specifieddensity at its initial eell'potential Upon raising the cell potentialrapid disintegration of both the coating and substrate resulted. Thisdemonstrates the superior life of the anode of this invention.

Sample A, an anode of this invention, was tested for an additional 200hours at 3000 a.s.f. without failure, indicating that the anode willlast at least twice as long as Sample C, which did not contain thecarbide.

EXAMPLE III Sample D is prepared as follows:

An aqueous paint composed (by weight) of 15% ruthenium chloride, 6.25%boron carbide (325 mesh powder), 8.54% titanium chloride solution(containing 20% TiCl and 70.11% water is applied to one side of apreviously prepared titanium sheet. The coated substrate is fired at 725C. for minutes in air. This procedure is repeated to make a totalapplication of 5 coats. coating contains an amount of ruthenium oxideequivalent to microinches of ruthenium coating. This coating hasexcellent adherence.

A reference anode similar to Sample B, described in Example I, isprepared, except that it had a 40 microinch Pt-Ir coating.

Using the laboratory scale diaphragm cell and procedure described inExample I with Sample D and the reference sample as the anodes, Sample Dis found to have a cell potential of 4.90 volts and an anode potentialof 1.230 volts, and the reference anode a cell potential of 4.85 voltsand an anode potential of 1.215 volts.

EXAMPLE IV Sample E is prepared using the same procedure as described inExample III, except that tungsten carbide in the form of a line powderis used instead of boron carbide in the formulation.

A reference anode having a low overvoltage 40 microinch coating of Pt-Iris prepared for comparative electrical performance in a chlorine cell,as previously described.

The anode of the present invention, Sample E, has a cell potential andovervoltage performance comparable to the typical conventional lowovervoltage Pt-Ir reference electrode. Sample E is found to have a cellpotential of 5.90 volts and an anode potential of 1.210 volts, and thereference electrode a cell potential of 6.00 volts and an anodepotential of 1.250 volts.

EXAMPLE V Sample F is prepared as follows:

A paint formulation composed (by weight) of 8.0% silicon carbide (as afine powder) 5.0% ruthenium chloride (40% Ru), and 87.0% 2-propanol isapplied to one side of a previously prepared titanium sheet. The coatedsubstrate is fired in air at 500 C. for 5 minutes. This procedure isfollowed a total of five times to give a coating composed of rutheniumoxide and silicon carbide containing an amount of Ru equivalent to a 7microinch Ru coating.

Sample F is used as an anode in a laboratory scale chlorine cell test,previously described, and compared with a low overvoltage Pt-Irreference anode similar to Sample B. Sample F is found to have a cellpotential of 4.2 volts and an anode potential of 1.20 volts, and thereference anode a cell potential of 4.1 volts and an anode potential of1.17 volts.

EXAMPLE v1 Sample G is prepared as follows:

An aqueous paint composed (by weight) of 2.00% silicon carbide (having aparticle size of less than 250 microns), 7.75%..ruthenium chloride(38.6% Ru), 6.67% Ludox HS (an aqueous colloidal hydrophilic solutioncontaining 30% SiO and 83.58% water is applied to one side of apreviously preparedtitanium sheet. The coated substrate is fired at 500C. for 5 minutes. This procedure is repeated an additional four times togive a coating compound of 50.0% RuO 25.0% SiC, and 25.0% SiO The totalweight of the coating applied is 0.0111 gram. This deposit contains anamount of Ru equivalent to a 7.2 microinch Ru coating. The coating hasexcellent coherence and adherence.

Sample H is prepared as follows:

An aqueous paint, similar to that used for Sample G except that no SiCis present, is applied to a titanium sheet. The paint is composed of (byweight) 17.5% ruthenium chloride, 10.0% Ludox HS (an aqueous, colloidal,hydrophilic silica solution containing 30% SiO' and 72.5% water. As inthe case of Sample G, five coats are applied. Each coat is fired at 550C. for 10 minutes. The total weight gain is 0.0096 gram. This coatingwas not as adherent or coherent as the coating of Sample G.

Sample G and H are used as anodes a laboratory scale chlorine cell test,described in Example I and the results are shown in Table III with thoseusing a reference anode, prepared similarly to Sample B.

The examples demonstrate that the anodes of this invention, having aruthenium oxide-carbide coating, have excellent electricalcharacteristics, comparable to conventional low overvoltage platinumcoated anodes, and they have long life. In addition the anodes of thepresent invention are lower in cost than the conventional anodes.

What is claimed is:

1. A method of preparing an electrolytic anode comprised of a corrosionresistant valve metal substrate and a thin adherent coating consistingessentially of ruthenium oxide and an inert carbide comprising:

(a) depositing on said substrate a coating containing the inert carbideas a fine powder in a liquid medium and ruthenium as a salt, metal, oroxide and (b) firing said coated substrate in an oxidizing atmosphere todrive off the liquid and form a coherent adherent coating on thesubstrate.

2. A method of claim 1 wherein the carbide is a compound of an elementselected from the group B, Si, Ti, Hf, V, Nb, Ta, Cr, Mo, W andcombinations thereof.

3. A method of claim 1 wherein the carbide is present as a powder havinga particle size of no greater than 250 microns.

4. A method of claim 3 'wherein at least some of the carbide particleshave a diameter no greater than 10 microns.

5. The method of claim 3 wherein the carbide slurry contains a rutheniumsalt dissolved in the liquid medium and the coated substrate is fired ata temperature in the range of about 400 to 800 C.

6. The method of claim 3 wherein the carbide slurry contains finelydivided ruthenium dispersed in an organic liquid and the coated substratis fired at a temperature in References Cited the range Of about 400 to800 C. M i

7. The method. of claim 3 wherein the carbide 'slu rry f contains finelydividedruthenium oxide and theccj altecl 313232283 i gfigfi g substrateis fired at a temperature of about 1000" C. V I U 8. A method of claim 3wherein the carbide is bQrQfi v ALFRED LEAVITT, Primary mi f carbide. VU 1 f v I 9. A method of Claim 3 wherein the cmbide is silicon M F: F QAsslstantfixamm carbide. I I v 10. Amethod 0f claim3wherein thebrbifleistungsten "f; US' CLVX'RA y carbide. r 1l7.-46 R; 46 FA,'1Q6 c,221;:04-29 F

